The present invention relates to a SMD(surface mount device)-type LED (light emitting diode) with high heat dissipation efficiency and high power, especially to a light emitting diode with separate structures for electricity conduction and heat dissipation. While being assembled, an external circuitry and a heat sink are connected with a light emitting diode chip in different directions.
Light emitting diode (LED) is a fine solid-state light source made of semiconductor material. The device that turns electricity into light features on the compact structure, with long lifetime, low driving voltage, fast reaction, and good shock resistance. It can also be applied in various appliances with light weight and compact design and is quite popular in our daily lives.
According to wavelength, LED is divided into visible LED and invisible LED. As to visible LED, the general LED and high brightness LED are differentiated by the brightness—1 candle/sq.cm. The former is applied to indoor display systems while the later is suitable for outdoor display such as center high-mounted stop lamp, outdoor LED display and traffic signs. By development of the manufacturing techniques of LED chip, the luminous efficiency of LED is also improved and a larger size LED chip is available. In accordance with higher electricity, luminous flux of a single high power LED chip is decades of lumen, even hundreds of lumen. Therefore, it is believed that LED will replace incandescent lamp as a new generation of light source. People in industry or research institute in various countries are dedicated to developing high-luminous LED chip and the related lamp design.
However, by the increasing of the electric current, the heat produced by the LED chip also rises. Thus how to dissipate heat effectively becomes an important issue in the package and application of high power LED in order to ensure light emitting efficiency and reliability of LED.
For solving the over-heat problem of high power LED, it is obviously a trend to connect a LED chip with a leadframe with larger size than conventional one as well as a heat sink with good thermal conductivity. Refer to U.S. patent application publication No. 65,902,352, applied by one of the American leading companies that manufacture high power LED-Lumileds on Feb. 2, 2001, a high power LED is attached on top of a metal base with high thermal conductivity so as to dissipate high heat produced by a LED chip working under normal operating current (such as 350 mA) rapidly. Under the limitation of requirement for downsizing of LED components, the volume and surface of the metal base are restricted so that the capacity for heat dissipation is not enough. Thus the metal base should be connected with an external heat sink with larger size for enhancing heat dissipation. Thus the device can be used for a long time under normal operating current (such as 350 mA). However, the connection direction of circuitry and the connection between the metal base and the external heat sink are in the same direction. In order to avoid the short circuit between the positive and negative electrodes on circuitry, there is an insulating layer on up surface of the external heat sink for connecting with the positive and negative electrodes of LED. The insulating layer not only reduces the thermal conductivity of the heat sink, but also raises the difficulty of processing that the circuitry is connected by soldering while the connection of heat sink is by other welding techniques.
A high power LED with similar design is also disclosed in US patent application publication No. 2004/0075100A1, dated Oct. 10, 2003, applied by Georg Bogner et al. The connection direction between the metal base and the external heat sink is the same with the connection direction of circuitry with the metal base. Thus when being applied, the device also has the same problems of heat dissipation and assembling as mentioned above.
Refer to Taiwanese patent application publication No. 488557, dated Jun. 13, 2001, applied by Lite-On Technology Corporation, a LED with a low thermal resistance is disclosed. Refer to FIG. 1A, a LED packaging structure is composed of a low thermal resistance LED with a heat dissipation plate 10′, a LED chip 12′ over the dissipation plate 10′, positive and negative electrodes of the LED chip 12′ connected with a printed circuit board 13′, and a protective cover 14′; a heat dissipation base 3′ connected with the dissipation plate 10′ of the LED with a low thermal resistance; a copper-clad laminates 2′ joined with the printed circuit board 13′ of the LED with a low thermal resistance. Therefore, the surface-to-air thermal resistance of the LED chip 12′ is reduced and the efficiency of LED is enhanced.
The structure for electricity conduction includes the printed circuit board 13′ connected with the copper-clad laminates 2′ while the heat dissipation structure is through the heat dissipation plate 10′ connected with the heat dissipation base 3′. Although the connection directions of structure for electricity conduction and the heat dissipation structure are different, the device still has the disadvantage that the heat dissipation plate 10′ is not pressed on the heat dissipation base 3′ firmly, the connection therebetween is not tight so that the heat dissipation efficiency has been affected. In another embodiment, shown in FIG. 1B, the upper and lower pieces of printed circuit boards are fastened by screws so that the heat dissipation plate is not connected with the heat dissipation base firmly. But the upper printed circuit board with low rigidity is easy to get deformed. Thus once the two side of the printed circuit boards are fastened by screws, it must have warp and this lead to the breakage or damage of the tiny gold wire connecting between the positive and negative electrodes of LED chip and the printed circuit board. The diameter of the gold wire is generally 0.03 mm. Therefore, the reliability of the LED has been affected badly.
Thus there is a need to provide a novel SMD-type LED with separate structure for electricity conduction and heat dissipation so as improve the above shortcomings of traditional SMD-type LED.